The Curious State of Rigor Mortis

Another day, another corpse! It’s all in a day’s work if you happen to be a forensic pathologist. Yet this one’s a little different: all the muscles are stiff and it won’t move. What’s brought this about? And, perhaps more importantly, what does it mean? Step inside our mortuary.

We’re going to examine this “stiffness” which affects most animals shortly after death. Rigor mortis is the name for this curious phenomenon; translated from Latin, it simply means the “Stiffness of Death”.

So what causes this rather peculiar state? We’ll have to peer under the skin to find out.


Stiff as a board: not really a state of grace

Rigor mortis can be essential in determining the approximate time, and in certain cases even the cause of death. It sets in at around two to three hours after death, and it’s presence is felt until after around sixty hours (depending on factors like ambient temperature and how rainy it is).

In the early 19th century, the French paediatrician Pierre Nysten developed an eponymous law which described that this “stiffness” generally develops in certain areas first (eyes and face) and gradually spreads to the extremities after death, following a set pattern. It’s certainly grotesque (but ever so characteristic of the French) and this too can be used to provide a rough guide to the time of death.

If the deceased has had a large amount of physical exertion prior to death, they may generally expend all their oxygen (and therefore ATP) prior to death (if for example, they are in a violent struggle to the death). This can mean that at the time of death, rigor mortis sets in instantly. This is referred to as a “cavaderic spasm”. The position that the individual is in at the time of the onset of this bizarre phenomenon may actually pinpoint a cause of death.

Something to remember, though: it’s not a foolproof indication of the time of death, and for this pathologists generally tend to look at insect activity around the corpse, stomach contents of the body and bodily temperature in determining a more precise time of death.


A rigor-ous study: the science behind it

All bodily movement is made from the contraction and relaxation of muscle. Muscle is made out of two types of fibres, actin (thin) and myosin (thick). When a muscle contracts, myosin sticks to the actin and pulls the muscle fibres together. When this happens in thousands of muscle fibres all at once, you get a large muscle contraction. The air you breathe in is used (in part) to make adenosine triphosphate (ATP for short), which you may remember is used in the energy cycle. The ATP attaches to myosin, forcing it to let go of the actin, and the muscle relaxes. This is how it works for living organisms, not for zombies.

In the case of a dead body, oxygen is no longer being respired by that individual, because last we checked — you need living, breathing lungs to get away with this. This means that the production of ATP has ceased and once the muscles have contracted they stay contracted. All that’s needed for rigor mortis to set in now is the initial contraction. Here’s how it happens:

ATP is also used to pump calcium ions out of the cells against a concentration gradient in a living animal. Once you’ve passed on, calcium ions in the extracellular tissue fluid flow down the concentration gradient and into the muscles, binding actin and myosin together, creating a pull in the muscles, and thus a dead body becomes rigid due to muscle contraction. No further ATP supply means no release and the muscles stay taut. This occurs after approximately three hours after death.


Your Turn: Have you heard the term rigor mortis used before? If so, where? Let us know what you think about today’s article in the comments. We’d love to hear from you.



Leave a Reply